Rotorcraft, such as helicopters, are supported vertically by a plurality of driven blades which rotate about a vertical axis. It will be appreciated that, since both the lift and propulsive force of the helicopter are supplied primarily through the large rotating blade system, it is advantageous to provide a main blade configuration which achieves a high lifting force at a given airspeed and which does not experience a high aerodynamic drag.
The prior art contains many examples of blades and airfoils attempting to achieve this high lift-low drag performance ideal, most notably U.S. Pat. No. 3,728,045 issued Apr. 17, 1973 to Balch, U.S. Pat. No. 4,142,837 issued Mar. 6, 1979 to de Simone and U.S. Pat. No. 4,569,633 issued Feb. 11, 1986 to Flemming, Jr.
The Balch airfoil, designated SC1095, provides an airfoil cross section which achieves both higher maximum lift and lower zero lift aerodynamic drag at high velocity conditions as compared to the then-existing airfoils. These improvements were realized by selectively shaping the airfoil surface for both higher lift and lower drag by delaying separation of the airflow boundary layer over the airfoil, as well as decreasing the local Mach number at high freestream velocities.
The de Simone airfoil, also referred to as the SC1095-R8 configuration, is characterized in the referenced patent as an improvement over the SC1095 airfoil wherein the airfoil upper surface is shaped to distribute the surface static pressure peak over a greater area thereby reducing the likelihood of airfoil boundary layer separation. The SC1095-R8 airfoil achieves higher maximum lift than the SC1095 configuration at lower velocities, but is subject to higher zero lift drag forces at high velocity conditions.
The Flemming, Jr. configuration is an improvement over the SC1095-R8 design and is particularly well adapted for use in high speed applications, such as in the radially outer tip portion of the main blade. The Flemming, Jr. airfoil, also termed the SSC-Axx family, further reduces the zero lift drag force at high air speeds by delaying the creation of shock waves in the local airflow. This further reduction in aerodynamic drag at high air speed is achieved at the expense of some maximum lift at lower velocity operation.
As will be apparent from considering the above mentioned references as a group, prior art blade designers combine several types of airfoil cross sections along the span of an individual blade in an attempt to maximize the lift and minimize the drag over the range of expected airflow velocities. For example, U.S. Pat. No. 4,248,572 issued Feb. 3, 1981 to Fradenburgh shows a single helicopter blade which utilizes the high lift SC1095-R8 airfoil configuration in the lower velocity central span region of the blade and the lower lift SC1095 airfoil section radially outward thereof in that portion of the blade which encounters higher air velocities. The Flemming, Jr. patent provides a still further modification wherein members of the SSC-Axx family of airfoils is used in the rotor tip portion due to its still greater resistance to shock wave formation at high airflow velocities.
As can be seen, the twin goals of high lift and low drag in prior art airfoils are exclusionary, leading designers to specify high lift airfoil configurations only in the lower speed regions of the rotorcraft blade while being content with reduced lift in the radially outer high speed portions in order to avoid excessive aerodynamic drag. What is needed is an airfoil configuration able to produce high lift without experiencing unacceptably high drag force under high airflow velocity operation.